3. 1. output: 1, 3, 5, 7, 9

      for(int i = 1; i < 10; i+=2)
         cout << i << ", ";
      cout << endl;

   2. output: 100, 90, 80, 70, 60, 50

      for(int i = 100; i > 49; i-=10)
         cout << i << ", ";
      cout << endl;

   3. output: 1, 4, 9, 16, 25

      for(int i = 1, j = 3; i < 26; i+=j, j+=2)
         cout << i << ", ";
      cout << endl;